The invention relates to a process for producing a structural component.
A process of the generic type is known, for example, from DE 197 11 829 C1. The process for producing a fiber-reinforced structural component which is known from this document comprises the steps of pressing a mixture of coated carbon fiber bundles, fillers and binder to produce a pressed body, pyrolysing this pressed body to form a carbon-containing preform, and infiltrating this preform with liquid silicon to form a fiber-reinforced matrix of silicon carbide (SiC matrix), which, depending on the process conditions used, may contain silicon residues. Structural components produced using this process have a ductility which is high for ceramic materials and a high resistance to wear and can be used at temperatures of up 1400xc2x0 C. However, if structural components of this type are exposed to such temperatures for a prolonged period under oxidizing conditions, the carbon fibers at the surface may be oxidized. When the surface is mechanically stressed, this may lead to the surface fibers becoming detached, leading to a rougher surface and a reduction in wear resistance.
DE 198 05 868 A1 describes a process in which pressing compounds containing fibers of different qualities are pressed in a plurality of laminar layers. After the infiltration with the silicon, the component obtained in this way, which is in the form of a brake disc, has a ductile middle layer and two more brittle but more oxidation-resistant outer layers. However, the highest mechanical load on the brake disc acts on the inner region of the ring which, for process reasons, is likewise formed by relatively ductile and brittle layers.
Accordingly, the invention is based on the object of improving the process described above in such a way that the oxidation of the carbon fibers at the surface is minimized and, at the same time, the ductility of the structural component is substantially retained.
The solution consists of a process having the features of the presently claimed invention.
The inventive process according to the present invention is distinguished by the fact that specific, different mixtures of fiber bundles, generally carbon fibers, fillers and binders are used to produce a pressed body. The mixtures substantially differ from one another with regard to the quality of the bundles of fibers, their length, the quantity of individual fibers therein and their coatings. One mixture contains relatively long fibers with a protective layer (mixture A), which largely prevents reaction between the silicon and the carbon fiber. As a result, the reinforcing effect of the fibers is optimally utilized in the component, leading to a ductile behavior of this component region (ductile regions). A further mixture contains, in addition to fillers and binders, fibers which are less well protected against the liquid, infiltrating silicon (mixture B). During the infiltration, these fibers are largely converted by the silicon to form a homogeneous, dense and oxidation-resistant but relatively brittle SiC layer (brittle regions). The separate position of the mixtures in the radial and axial directions in a press mold therefore allows the materials properties of the resulting component to be adapted to the mechanical and frictional demand imposed on the component.
During the filling of a press mold with the mixtures A and B, separating features are arranged in a press mold, preventing the mixtures A and B and/or further mixtures from mingling with one another. The separating feature may be designed in the form of metal sheets or foils. The sheets or foils are shaped in such a way that the mixtures A and B can be arranged separately from one another in all spatial directions. After the filling, the separating features are removed, and the mixtures are pressed to form the pressed body. The separate regions of the mixtures A and B are retained in the pressed body.
To make surface regions of the structural component particularly resistant to wear, the mixtures are arranged in such a way in the press mold that the mixture B lies at the frictionally loaded friction surfaces of the component.
To increase the ductility of the mechanically loaded component regions, mixture A is arranged in such a way in the press mold that it is distributed in the core of the component and in the inner ring, since this is where particularly ductile materials properties are required.
The material produced by the process is a gradient material. This means that there is no exact contact layer between the component regions. This is attributable to the mingling of the pressing compounds at their boundary surfaces during the pressing. The gradual transitions between the regions are particularly advantageous for avoiding delamination or desired breaking points.
The proportion of SiC in mixture B used at the surface regions is higher than in the regions in the core and in the inner ring (mixture A), where the fibers are in the form of carbon. On account of the different material compositions and their different expansion coefficients, thermal stresses may be generated. To reduce possible thermal stresses, it is expedient to form grooves in the brittle regions of the surface, which grooves are of approximately the same depth as the brittle regions.
A further solution consists in an inventive process according to the present invention. In this process, the finished structural component, which is produced almost completely from a mixture A, is impregnated, at the frictionally loaded surfaces, with a medium which forms an oxidation-resistant layer during further treatment. This medium is a liquid medium which is capable of impregnation and contains a base material for the oxidation-resistant layer and a solvent. A low wetting angle with respect to the surface of the structural component and a low viscosity are advantageous for the impregnability of the medium. The advantage of this process consists in the fact that the entire component can be formed by ductile regions, and the fiber bundles at the surface are additionally protected against oxidation at high temperatures.
Polymers which produce compounds comprising silicon, oxygen and carbon during a heat treatment at below 200xc2x0 C. are suitable as base material for the oxidation-resistant layer. The impregnation may take place a number of times, in order to achieve optimum results.
Furthermore, SiC-containing adhesives which are dissolved in a solvent and the viscosity of which, which influences the impregnation behavior, can be adjusted by means of the solvent content are suitable for impregnation. In this case, multiple impregnation is once again particularly advantageous.
The materials for impregnation of the surface generally impregnate spontaneously through the action of the capillary forces. To accelerate the impregnation process, it is expedient for this process to be assisted by the use of an elastic bar which is drawn over the surface. In this way, the pressure on the surface is locally increased. The use of a squeegee, as is also used in screen printing, is particularly suitable for this purpose.
A further inventive solution according to the present invention is the production of a layer which substantially comprises silicon carbide (SiC layer) on the component surface. For this purpose, during the production of the structural component, a layer of pyrolysable material (pyrolysable layer) is at least partially applied to the pressed body or to the porous preform or to the structural component, preferably to the porous preform. The term pyrolysable material is understood as meaning an organic material, such as for example pitches or phenolic resins, which can be reduced to form carbon. Furthermore, in its starting form the pyrolysable material may contain carbon in all modifications and forms (e.g. in fiber or powder form), as well inorganic fillers, such as for example SiC. The pyrolysis of the pyrolysable layer takes place either during a process-related heat treatment (pyrolysis of the pressed body or infiltration of the preform), or an additional process step is introduced for this purpose. After the infiltration with silicon, the regions of the pyrolysable layer have been converted into an SiC layer, which protects the carbon fibers below from oxidation at high temperatures and, moreover, is particularly wear-resistant. The SiC layer is securely joined to the component surface. Furthermore, the SiC layer may have a pattern of fine hairline cracks, which is advantageous for reducing possible stresses in the SiC layer.
To reduce materials costs, the pyrolysable layer may, in addition to the above-mentioned constituents, contain milling residues, which are produced during the machining of the preform, and/or SiC. Depending on requirements and availability, this mixing may take place in different combinations. The pyrolysis may take place during the pyrolysis of the pressed body to form the preform if process features, such as for example the condition of the pyrolysis furnace or of the furnace for infiltration require this to be the case. However, it is particularly preferable for the pyrolysable layer to be applied to preform and to be pyrolysed during the heating for infiltration with the silicon, since this results in the smallest reaction-related change in volume in the layer.
In a further embodiment, the pyrolysed layer comprises caramelized sugar, which is likewise preferably pyrolysable during the heating for silicon infiltration and is infiltrated with silicon in the same process step. After the infiltration, the result is a dense SiC layer which protects the fiber bundles below from oxidation. This variant of the SiC layer is particularly expedient on account of the low raw materials costs.
All the SiC layers described have a thickness of from 0.2 to 5 mm. In practice, is has been found that a layer thickness of between 0.5 mm and 2.5 mm is particularly favorable with regard to the protection against oxidation and requires the lowest outlay on process engineering.
All the processes which have been described to date are particularly expedient if the component is a brake disc which is already in use, for example, in automotive engineering and in railway vehicle engineering.
In this application, particularly high temperatures occur at frictional faces of the surface, which is precisely where a particularly high resistance to wear is required.
On the other hand, a particularly high ductility of the supporting material is required in the core layer of the disc and, in particular, in the inner region of the ring. Particularly for a brake disc, the invention offers an excellent combination of particularly wear-resistant and particularly ductile regions while guaranteeing all other properties which are required for this component.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.